Method for forming a composite structure

ABSTRACT

The present invention related to a method for providing a composite structure and the composite structure formed thereby. The method broadly comprises the steps of: providing a core structure formed from a non-metallic composite material in a substantially uncured state; applying a fluoroelastomer film in a substantially uncured state over surface portions of said core structure; placing a metallic structure over a portion of said core structure, said metallic structure having inner surfaces which contacts said fluoroelastomer film; and molding said composite structure under conditions of heat and pressure for a time sufficient to co-cure said fluoroelastomer film and said non-metallic core structure material and to form a bond between said fluoroelastomer film and said inner surfaces of said metallic structure. The method of the present invention may be used to manufacture original metal/composite structures as well as to refurbish metal/composite structures such as fan exit guide vanes for jet engines.

BACKGROUND OF THE INVENTION

The present invention relates to a method for providing a compositestructure having improved protection against erosion and foreign objectdamage and the composite article formed thereby. The method of thepresent invention has particular utility in the manufacture andrefurbishment of airfoil structures.

Composite airfoil structures such as the fan exit guide vane on a jetengine are subject to erosion from atmospheric effects as well as fromdamage resulting from the impingement of foreign objects on the leadingedge of the vane. Efforts have been made to find suitable techniques forrepairing these airfoil structures. There remains a need however for amethod of refurbishment which yields a well bonded structure having thedesired protection against erosion and foreign object damage.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for refurbishing the leading edge of a composite structure suchas an airfoil structure.

It is a further object of the present invention to provide a method asabove which is economically beneficial and eliminates the need forsecondary bonding operations.

It is yet a further object of the present invention to provide a methodas above which improves the erosion and foreign object damage protectionon the leading edge of the composite structure.

It is still a further object of the present invention to provide amethod as above which also lends itself to the formation of originalmetal/composite structures.

It is yet a further object of the present invention to provide a wellbonded composite structure.

The foregoing objects are attained by the method of the presentinvention.

In accordance with the present invention, a novel method for forming acomposite structure is presented. The method comprises the steps of:providing a core structure formed from a non-metallic material in anuncured or substantially uncured state; applying a fluoroelastomer film,also preferably in an uncured or substantially uncured state and havinga cure temperature which closely matches the cure temperature of thecore structure material, over surface portions of said core structure;placing a metallic structure over a portion of said core structure sothat inner surfaces of said metallic structure contact saidfluoroelastomer film; and molding said composite structure underconditions of heat and pressure for a time sufficient to co-cure saidnon-metallic core structure material and said fluoroelastomer film andto form a bond between said metallic structure and said fluoroelastomerfilm. In a preferred embodiment, a temperature of up to about 360° F.(182° C.) and a pressure of from about 100 psi (7 kg/cm²) to about 1000psi (70 kg/cm²) are applied to the composite structure for about 30minutes during the co-curing and bonding operation.

It has been found that a particularly strong bond is formed during themethod of bonding if the metallic structure is prepared by cleaning theinner surfaces with a solvent such as an isopropyl alcohol solution,etching the metallic structure in a ferric chloride acid etchingsolution to remove unwanted oxides and other deleterious materials fromthe inner surfaces, and priming said inner surfaces of said metallicstructure to prevent the creation of new oxides and improve the bondingcharacteristics of the surfaces. The priming step preferably comprisesapplying an epoxy resin solution containing an insoluble corrosioninhibitor such as strontium chromate to said surfaces.

The method of the present invention has been found to have particularutility in the refurbishment of airfoil structures such as fan exitguide vanes used in jet engines. It also has utility in the manufactureof original equipment and the formation of metal/composite structures.The method of the present invention has been found to be excellenttechnology for bonding to stainless steel--something which has beenquite difficult in the past.

Other details of, as well as other objects and advantages attendant to,the method of the present invention are set forth in the followingdetailed description and the accompanying drawing(s) wherein likereference numerals depict like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross sectional view of a portion of a compositeairfoil structure;

FIG. 2 illustrates an enlarged cross sectional view of the leading edgeof a composite structure;

FIG. 3 is a graph showing the results of a ballistic test performed onfan exit guide vanes having wire mesh and stainless steel leading edges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to the drawings, FIGS. 1 and 2 illustrate a leading edgeportion of a composite structure 10, such as a fan exit guide vane,formed using the method of the present invention. The compositestructure 10 includes a core structure 12 formed from a non-metalliccomposite material, preferably in an uncured or substantially uncuredstate, a layer 14 of fluoroelastomer film material, also preferably inan uncured or substantially uncured state, having a cure temperaturesubstantially close to the cure temperature of the core structure, and ametallic structure 16 placed over a leading edge portion 18 of the corestructure.

When the composite structure 10 is to be used as a fan exit guide vane,the core structure 12 may be formed from a composite graphite--epoxymaterial having a cure temperature of about 350° F. (177° C.). Thematerial may be in an uncured or substantially uncured state and mayconsist of one or more core plies 20 and one or more shell plies 22.Typically, all of the plies 20 and 22 will be formed from the samecomposite material. The core structure 12 may have any desired shape andany desired thickness. When the composite structure is to be used as anairfoil structure such as a fan exit guide vane, the core structure 12preferably has an aerodynamic shape.

The fluoroelastomer film layer 14 may be formed from a VITON film havinga cure temperature of about 350° F. (177° C.), which film may be in anuncured or substantially uncured state. The film layer has a typicalthickness of about 0.008 to about 0.015 inches (about 0.02 to about0.038 centimeters). A suitable VITON film is a fluoroelastomer productmanufactured by Eagle Elastomer of Cuyahoga Falls, Ohio. (VITON is atrademark of duPont.) The fluoroelastomer film layer 14 acts as anadhesive and may be applied to the surface 24 of the core structure 12using any suitable technique known in the art. For example, thefluoroelastomer film material may be brushed or otherwise painted ontothe surface 24 of the leading edge of the core structure. If desired,the fluoroelastomer film layer may be applied so that it extends toportions of the surface 24 beyond the leading edge of the structure 10.In order to provide improved protection against erosion and againstforeign object damage, a metal structure or metal sheath 16 is placedover the leading edge portion 18 of the composite structure. The metalstructure 16 may be formed from an iron based alloy material such as AMS5510 or a nickel based alloy material such as AMS 5536 or AMS 5599.

It is preferred that the metal structure 16 be a solid structure becausesolid structures provide better protection against erosion and foreignobject damage. When used as a leading edge structure, the metalstructure 16 typically will have a V-shape and a thickness in the rangeof from about 0.005 to about 0.012 inches (about 0.013 to about 0.03centimeters).

To form a composite structure, the fluoroelastomer adhesive material 14is first applied to outer surface portions 24 of the core structure 12.As discussed above, the adhesive material may be applied in film form onthe surface portions 24.

Prior to being placed over the adhesive layer 14, the metal leading edgestructure 16 is subjected to a critical preparation process whichactivates inner surfaces 26 of the metal structure 16 and therebygreatly improves the bond that is formed between the metal structure 16and the core structure 12. The critical preparation process comprisescleaning inner surfaces 26 of the metal structure 16, etching the innersurfaces 26 so as to remove any oxides and to provide a fresh surfacefor bonding, and priming the inner surfaces 26 so as to prevent theformation of oxides and thereby improve the peel strength of the bondthat is ultimately formed.

In a preferred preparation process, the inner surfaces 26 of the metalstructure 16 are first wiped clean using a clean, unsized cheeseclothdampened with any suitable solvent such as reagent grade isopropylalcohol or acetone. After wiping, the inner surfaces may be either airdried at ambient temperature or oven dried at a maximum temperature of250° F. (121° C.). The drying time should be sufficient to remove anytraces of the cleaning solvent which may be detrimental to the bondingprocess.

Thereafter, the inner surfaces 26 are etched using a ferric chlorideacid etch solution to remove any oxides on the surfaces and to provide afresh surface for bonding. Prior to the application of the etchingsolution, the metallic structure 16 is immersed in an alkaline cleaningsolution such as a solution containing an alkaline cleaner sold underthe trade name Blue Gold Industrial Cleaner or an equivalent alkalinecleaner in a volume of 5 oz. per gallon. The metallic structure isimmersed in the solution for at least 5 minutes. The solution ispreferably maintained at a temperature in the range of about 120° F.(49° C.) to about 160° F. (71° C.). After removal from the alkalinecleaning solution, the metallic structure 16 is rinsed in cold water for20 to 40 seconds.

Etching of the inner surfaces 26 of the metallic structure is carriedout by immersing the metallic structure in a ferric chloride acidetching solution. The etching solution may be made up by providing 80gallons (302.4 liters) of hydrochloric acid and adding 135 lbs. (61.3kgs) of anhydrous ferric chloride in small increments and maintainingthe mixing tank temperature below 120° F. (49° C.). After the solutionhas cooled to a temperature of about 90° F. (32° C.), 2 gallons (7.56liters) of nitric acid and 11 gallons (41.6 liters) of water are added.The etching operation is preferably carried out at room temperature forabout 14 to 16 minutes. If necessary, the inner surfaces 26 may beabrasively or grit blasted with 240 mesh aluminum oxide at 20 psi (1.4kg/cm²) pressure prior to initiating etching.

After the etching operation has been completed, the metallic structureis rinsed in cold water for 20 to 40 seconds and thereafter power waterflushed with cold water. The surfaces 26 are then inspected for smut orother deleterious particles. If smut or other deleterious particles arefound, they are preferably manually removed with a suitable wipe.Thereafter, the metallic structure is subjected to another power waterflush with cold water. After rinsing, the metallic structure 16 ispreferably placed in an oven and dried at a temperature in the range ofabout 140° F. (60° C.) to about 160° F. (71° C.) for a time period inthe range of from about 14 to about 16 minutes.

As a final preparation step, the inner surfaces 26 are primed so as toprevent oxidation of the surfaces and improve bonding. The primingoperation consists of applying an epoxy resin solution containing aninsoluble corrosion inhibitor such as strontium chromate to the innersurfaces 26. Any suitable technique known in the art may be used toapply the priming epoxy resin solution to the surfaces 26. The primermust be cured at the suitable conditions (such as air at 250° F. (121°C.) for 30 minutes).

After the surface preparation treatment has been completed, the metallicstructure 16 is placed over the leading edge portion of the corestructure 12 with the inner surfaces 26 of the metallic structurecontacting the fluoroelastomer adhesive material 14. The entirecomposite structure is then placed in a mold such as a compression mold(not shown) and subjected to heat and pressure for a time period of 30minutes to effect co-curing of the epoxygraphite material forming thecore structure 12 and the fluoroelastomer adhesive material 14 and tocreate a relatively strong bond between the etched and primed innersurfaces 26 of the metallic structure 16 and the fluoroelastomeradhesive material 14. The pressure which is applied to the compositestructure should preferably be in the range of from about 100 psi (7kg/cm²) to about 1000 psi (70 kg/cm²). The temperature which is appliedto the composite structure in the mold should not exceed 360° F. (182°C.). Typically, a temperature of about 350° F. (177° C.) is used duringthe co-curing and bonding operation.

It has been found that the combination of cleaning, etching and primingsteps performed on the metallic structure 16 are critical to achieving asuccessful bond during the molding operation. It also has been foundthat the co-curing of the core structure material and the adhesive layermaterial eliminates the need for a secondary bonding operation. As aconsequence of this, the cost of producing the composite structure isgreatly reduced. The end result of the method of the present inventionis a well bonded metal sheath on the leading edge of a compositestructure, such as a fan exit guide vane, which improves erosion andforeign object damage protection.

It has been known in the industry for some time that bonding tostainless steel is very difficult. The method of the present inventionallows bonding to stainless steel materials such as AMS 5510 and theaccomplishment of peel strengths in the range of about 20-70 lbs/inchwidth (3.58-12.5 kg/cm width) with failures occurring primarily withinthe VITON adhesive layer.

It has been found that a leading edge formed using a stainless steelmaterial such as AMS 5510 is particularly advantageous. This has beendemonstrated using a small particle ballistic test. In this test, 0.125inch (0.32 centimeter) diameter steel ball bearings were fired at theleading edge of fan exit guide vanes having either a 0.012 inch (0.03cm) thick wire mesh leading edge or a 0.010 inch (0.025 cm) thickstainless steel leading edge. FIG. 3 shows the improvement in smallparticle impact that the stainless steel leading edge provides ascompared to a wire mesh leading edge. The wire mesh leading edge waspunctured at particle energies of 0.25 ft-lb. (0.035 kg-m). Thestainless steel leading edge was impacted with particles having energylevels up to 0.6 ft-lb. (0.083 kg-m) with no puncture. The stainlesssteel leading edge provides approximately a 40% reduction in dent depthwhen compared to the wire mesh leading edge at an energy of 0.25 ft-lb(0.035 kg-m). More important than dent depth, when compared to the wiremesh leading edge, the stainless steel leading edge demonstrated amargin of greater than 140% in the energy required to puncture.

In order to demonstrate the improved peel strengths obtainable using themethod of the present invention attributable to the ductile nature ofthe fluoroelastomer film, a number of composite material test specimenswere removed from the leading edge of molded fan exit guide vanes. Eachtest specimen consisted of a composite structure having anepoxy-graphite core material in an uncured state, an intermediate layerof a VITON adhesive material in an uncured state, and a layer of AMS5510 stainless steel material, which had been subjected to thepreparation process described hereinabove, placed over the layer ofadhesive material. Each test specimen was placed in a vane compressionmold and subjected to a pressure of 500 psi (35 kg/cm²) and atemperature of 350° F. (177° C.) for 30 minutes to co-cure the corematerial and the VITON adhesive material and to bond the stainless steellayer to the VITON material layer.

Several of the test specimens were subjected to different conditions.One test specimen was subjected to humidity conditions (140° F. (60°C.)/95%RH/14 days). Two test specimens were aged at 250° F. (121° C.)for 500 hours. Another two test specimens were thermal cycled attemperatures in the range of 0° F. (-18° C.) to 250° F. (121° C.) for100 cycles. Thereafter, the peel strength for each specimen wasdetermined as per ASTM D3167. It was found that: as fabricated testspecimens had peel strengths at 75° F. (24° C.) in the range of 16 to 32lbs/inch (2.86 to 5.73 kg/cm) width; the humidity tested specimen had apeel strength at 75° F. (24° C.) of 46 lbs/inch (8.23 kg/cm) width; theaged test specimens had peel strengths at 75° F. (24° C.) of 27 lbs/inchwidth (4.83 kg/cm width) and 41 lbs/inch width (7.34 kg/cm width); andthe cycled test specimens had peel strengths at 75° F. (24° C.) of 18lbs/inch width (3.72 kg/cm width) and 21 lbs/inch width (3.76 kg/cmwidth).

Testing has also shown that the fatigue strength of composite fan exitguide vanes with a stainless steel leading edge formed in accordancewith the method of the present invention are greater than that ofcomposite fan guide exit vanes formed in accordance with prior arttechniques and a wire mesh leading edge.

While the core structure 12 is preferred to be in an uncured orsubstantially uncured state, the method of the present invention can beperformed with a cured core material such as a cured graphite-epoxymaterial.

While the method of the present invention has been described in thecontext of putting a leading edge on an airfoil structure, it should berecognized that the method could be used in general metal to compositematerial bonding applications and should therefore not be limited to theembodiments shown herein. It is also applicable to other metals(titanium, aluminum, nickel) with appropriate surface preparations foradhesive bonding. It should also be recognized that the method hasutility in original equipment manufacture processes as well as repairprocesses.

It is apparent that there has been provided in accordance with thisinvention a metal/composite bonding method which fully satisfies theobjects, means, and advantages set forth hereinbefore. While theinvention has been described in combination with specific embodimentsthereof, it is evident that many alternatives, modifications andvariations will be apparent to those skilled in the art in light of theforegoing description. Accordingly, it is intended to embrace all suchalternatives, modifications, and variations as fall within the spiritand the broad scope of the appended claims.

What is claimed is:
 1. A method for providing a composite structurecomprising the steps of:providing a core structure formed from anon-metallic composite material in an uncured state; applying afluoroelastomer film in an uncured state over surface portions of saidcore structure; placing a metallic structure over a portion of said corestructure, said metallic structure having inner surfaces which contactsaid fluoroelastomer film; and molding the assembly of said corestructure, said fluoroelastomer film, and said metallic structure underconditions of heat and pressure for a time sufficient to co-cure saidfluoroelastomer film and said non-metallic core structure material andto form a bond between said fluoroelastomer film and inner surfaces ofsaid metallic structure.
 2. The method of claim 1 wherein said moldingstep is carried out a temperature of up to about 360° F. and a pressureof about 100 psi to about 1000 psi for about 30 minutes.
 3. The methodof claim 1 further comprising:activating the inner surfaces of saidmetallic structure prior to said placing step; and said activating stepcomprising cleaning said inner surfaces and etching said inner surfacesso as to remove unwanted oxides and other deleterious materials.
 4. Themethod of claim 3 wherein said cleaning step comprises wiping said innersurfaces with an isopropyl alcohol solution.
 5. The method of claim 3wherein said etching step comprises etching said metallic structure withsaid inner surfaces in a ferric chloride acid etching solution.
 6. Themethod of claim 1 wherein said core structure providing step comprisesproviding a composite structure formed from a graphite-epoxy material inan uncured state.
 7. The method of claim 1 wherein said film applyingstep comprises applying a film of a fluoroelastomer based on a copolymerof vinylidene fluoride and hexafluoropropylene or a terpolymer ofvinylidene fluoride, hexafluoropropylene and tetrafluoroethylene in anuncured state over said surface portions of said core structure.
 8. Themethod of claim 1 wherein said metallic structure placing step comprisesplacing a leading edge structure formed from an iron-based alloy or anickel based alloy over a leading edge portion of said core structure.9. A method for providing a composite structure comprising the stepsof:providing a core structure formed from a non-metallic compositematerial in an uncured state; applying a fluoroelastomer film in anuncured state over surface portions of said core structure; placing ametallic structure over a portion of said core structure, said metallicstructure having inner surfaces which contact said fluoroelastomer film;molding said composite structure under conditions of heat and pressurefor a time sufficient to co-cure said fluoroelastomer film and saidnon-metallic core structure material and to form a bond between saidfluoroelastomer film and said inner surface of said metallic structure;activating the inner surfaces of said metallic structure prior to saidplacing step; and said activating step comprising cleaning said innersurfaces and etching said inner surfaces so as to remove unwanted oxidesand other deleterious materials and further comprising priming saidinner surfaces by applying an epoxy resin solution containing aninsoluble corrosion inhibitor to said inner surfaces so as tosubstantially prevent the formation of unwanted oxides on and to improvethe bonding characteristics of said inner surfaces.
 10. A method formanufacturing a fan exit guide vane comprising:providing a corestructure formed from a graphite epoxy material; applying a film of afluoroelastomer based on a copolymer of vinylidene fluoride andhexafluoropropylene or a terpolymer of vinylidene fluoride,hexafluoropropylene and tetrafluoroethylene to surface portions of saidcore structure; placing a metallic leading edge structure over leadingedge portions of said core structure covered with said fluoroelastomerfilm; placing the assembly of said core structure, said fluoroelastomerfilm, and said metallic structure in a mold and co-curing said corestructure and said fluoroelastomer film material and bonding saidmetallic leading edge structure to said core structure by applying heatand pressure to the assembly for a time sufficient to effect saidco-curing and said bonding.
 11. The method of claim 10 wherein saidleading edge structure placing step comprises placing a stainless steelleading edge structure over said leading edge portion of said corestructure.
 12. A method for manufacturing a fan guide exit vanecomprising:providing a core structure formed from a graphite epoxymaterial; applying a film of a fluoroelastomer based on a copolymer ofvinylidene fluoride and hexafluoropropylene or a terpolymer ofvinylidene fluoride, hexafluoropropylene and tetrafluoroethylene tosurface portions of said core structure; placing a metallic leading edgestructure over leading edge portions of said core structure covered withsaid fluoroelastomer film; co-curing said core structure and saidfluoroelastomer film material and bonding said metallic leading edgestructure to said core structure by placing said core structure withsaid fluoroelastomer film and said overlaid metallic leading edgestructure in a mold and applying heat and pressure for a time sufficientto effect said co-curing and said bonding; said leading edge structureplacing step comprising placing a stainless steel leading edge structureover said leading edge portion of said core structure; pretreating saidstainless steel leading edge structure prior to placing it over saidleading edge portion of said core structure; and said pretreating stepcomprising wiping inner surfaces of said stainless steel leading edgestructure with an isopropyl alcohol solution, etching said stainlesssteel leading edge structure in a ferric chloride etching solution atroom temperature for a time period in the range of about 14 to about 16minutes, and thereafter priming said inner surfaces with an epoxy resinsolution containing an insoluble corrosion inhibitor.
 13. The method ofclaim 12 wherein said co-curing and bonding step comprises applying atemperature of up to about 360° F. and a pressure of about 100 psi toabout 1000 psi for a time period of up to about 30 seconds.
 14. A methodfor fabricating a fan exit guide vane comprising:providing a corestructure formed from a graphite epoxy material; applying an adhesivefilm of a fluoroelastomer based on a copolymer of vinylidene fluorideand hexafluoropropylene or a terpolymer of vinylidene fluoride,hexafluoropropylene and tetrafluoroethylene to surface portions of saidcore structure; placing a stainless steel leading edge structure overleading edge portions of said core structure covered with said adhesivefilm; and placing the assembly of said core structure, said adhesivefilm, and said stainless steel leading edge structure in a mold andco-curing said core structure and said adhesive film and bonding saidstainless steel leading edge structure to said core structure byapplying heat and pressure to the assembly for a time sufficient toeffect said co-curing and said bonding.
 15. The method of claim 14wherein said co-curing and bonding step comprises applying a temperatureof up to about 360° F. and a pressure of about 100 psi to about 1000 psifor a time period of up to about 30 seconds.
 16. A method forfabricating a fan exit guide vane comprising:providing a core structureformed from a graphite epoxy material; applying an adhesive film of afluoroelastomer based on a copolymer of vinylidene fluoride andhexafluoropropylene or a terpolymer of vinylidene fluoride,hexafluoropropylene and tetrafluoroethylene to surface portions of saidcore structure; placing a stainless steel leading edge structure overleading edge portions of said core structure covered with said adhesivefilm; co-curing said core structure and said adhesive film and bondingsaid stainless steel leading edge structure to said core structure byplacing said core structure with said adhesive film and said overlaidstainless steel leading edge structure in a mold and applying heat andpressure for a time sufficient to effect said co-curing and saidbonding; pretreating said stainless steel leading edge structure priorto placing it over said leading edge portion of said core structure; andsaid pretreating step comprising wiping inner surfaces of said stainlesssteel leading edge structure with an isopropyl alcohol solution, etchingsaid stainless steel leading edge structure in a ferric chloride etchingsolution at room temperature for a time period in the range of about 14to about 26 minutes, and thereafter priming said inner surfaces with anepoxy resin solution containing an insoluble corrosion inhibitor.